Make URLs consistent

[maroth]

Make all internal links to challenges use the version with the slug. remove the version without the slug from urls.py.

For example, from the organization details page, the links to challenges use the old way without the slug.

Do this for organization names to: remove the version of the URL without the slug from urls.py and make all links use the version with the URL.

[maroth]

There are still many places in the code where the URLs in the accounts app are hardcoded. Remove all hardcoded URL and replace them with ones generated from the URLS.py file.